U.S. patent number 5,552,705 [Application Number 08/457,535] was granted by the patent office on 1996-09-03 for non-obtrusive weapon detection system and method for discriminating between a concealed weapon and other metal objects.
Invention is credited to George V. Keller.
United States Patent |
5,552,705 |
Keller |
September 3, 1996 |
Non-obtrusive weapon detection system and method for discriminating
between a concealed weapon and other metal objects
Abstract
A non-obtrusive, non-threatening weapon detection system for
discriminating between a concealed weapon and other metal objects.
The system is designed for a high probability of detection with a
low false alarm rate. When the subject system is in use, a person
under surveillance need not be aware that he or she is being
monitored nor is the person's privacy invaded. The system includes
the use of a magnetic field generator for transmitting low
intensity electromagnetic step pulses causing eddy currents to flow
in any metal object carried by the person. The eddy currents
scatter a signal that is detected by one or more fast response
magnetic field sensors. The eddy currents excited in the metal
body, which may be called a target, by the leading edges of the
transmitted step pulses take the form of an exponentially decaying
transient immediately following the step pulses abrupt initial
rise. This "decay curve" provides a basic observable, namely, a
time constant of the current decay. The decay curve is analyzed by
an appropriate means such as a preprogrammed computer, to determine
accurately the time constant of the decay curve. The processed
information can then be compared with similar values of the time
constant contained in a stored data base to predict the nature of
the target. The analyzed information allows an observer to identify
the target as being threatening or non-threatening.
Inventors: |
Keller; George V. (Golden,
CO) |
Family
ID: |
26791204 |
Appl.
No.: |
08/457,535 |
Filed: |
June 1, 1995 |
Current U.S.
Class: |
324/239; 324/243;
340/551 |
Current CPC
Class: |
G01V
3/105 (20130101); G06K 9/3241 (20130101) |
Current International
Class: |
G01V
3/10 (20060101); G06K 9/32 (20060101); G01N
027/72 (); G01R 033/12 (); G08B 013/24 () |
Field of
Search: |
;324/239,240,241,242,243,326,327,328,329 ;340/552,551 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3707672 |
December 1972 |
Miller et al. |
4894619 |
January 1990 |
Leinonen et al. |
|
Primary Examiner: Snow; Walter E.
Attorney, Agent or Firm: Crabtree; Edwin H. Pizarro; Ramon
L. Margolis; Donald W.
Claims
The embodiments of the invention for which an exclusive privilege
and property right is claimed are defined as follows:
1. A non-obtrusive weapon detection system for detection of and
discrimination between a concealed weapon and other metal objects,
the system comprising:
a transmitter for transmitting low intensity electromagnetic step
pulses and causing eddy currents to flow in a metal object under
observation, the eddy currents creating a plurality of scattered
signals sufficient to accurately plot a "decay curve" due to the
relaxation of the eddy currents;
a receiver for detecting the scattered signals from the eddy
currents; and
data processing and computing equipment means connected to said
receiver, said data processing and computing equipment means
digitally processing the scattered signals for defining a time
constant of the decay curve, the processed information of the time
constant of the decay curve providing an accurate determination of
a relationship to the conductive properties of the object under
surveillance and the size and shape of the object using appropriate
mathematical equations, the processed information allowing an
observer to identify objects that are threatening or
non-threatening.
2. The weapon detection system as described in claim 1 wherein said
transmitter transmits repeated step pulses causing eddy currents to
flow in the metal object under observation and creating additional
scattered signals which are digitally processed and synchronously
averaged with the scatter signals for defining the time constant of
the decay curve and providing increased accuracy in the
determination of the object under observation.
3. The weapon detection system as described in claim 2 wherein the
scattered signals are synchronously stacked and replotted, ie.
algebraically transformed, by using a logarithm of the scattered
signals strength and creating an exponential decay curve as a
straight line, with the slope of the straight line being the time
constant.
4. The weapon detection system as described in claim 1 wherein said
data processing and computing equipment includes an embedded
analog-to-digital (A/D) converter linked directly to a
preprogrammed central processing unit (CPU) for synchronous
averaging and linear filtering to yield the decay curve in a range
of 1 to 10 microseconds following completion of the transmission of
the step pulses, the time of transmission of the step pulses in a
range of 0.1 to 2 seconds.
5. The weapon detection system as described in claim 1 wherein said
data processing and computing equipment means is used for comparing
the time constant of the decay curve of the metal object under
observation with similar values of time constants of other metal
objects contained in a data base of said data processing and
computing equipment means to predict the nature of the metal object
under observation and allow an observer to identify the metal
object as being threatening or non-threatening.
6. The weapon detection system as described in claim 1 wherein said
transmitter transmits step pulses causing eddy currents to flow in
more than one metal object under observation and at the same time,
said data processing and computing equipment means digitally
processing scattered signals for defining time constants of decay
curves of the metal objects thereby providing an accurate
determination of a relationship to the conductive properties of the
metal objects under surveillance and the sizes and shapes of each
metal object.
7. The weapon detection system as described in claim 1 wherein said
receiver is a fast response magnetic field sensor.
8. The weapon detection system as described in claim 1 wherein said
receiver is a plurality of fast response magnetic field
sensors.
9. The weapon detection system as described in claim 1 wherein said
transmitter is a magnetic field generator.
10. A non-obtrusive weapon detection system for detection of and
discrimination between a concealed weapon and other metal objects,
the system comprising:
a transmitter for transmitting low intensity electromagnetic step
pulses and causing eddy currents to flow in a metal object under
observation, the eddy currents excited by a transient associated
with a rise-time of the step pulses and with the eddy currents
relaxing as soon as excitation is over and creating scattered
signals to plot accurately a "decay curve" of eddy current
relaxation;
a receiver for receiving the scattered signal from the eddy
currents; and
a computer connected to said receiver and used to digitally process
scattered signals, the scattered signals synchronously stacked and
replotted, ie. algebraically transformed, by using a logarithm of
the scattered strength and creating an exponential decay curve as a
line, with the Slope of the straight line being the time constant,
the processed information of the time constant of the decay curve
providing an accurate determination of a relationship to the
conductive properties of the object under surveillance and the size
and shape of the object using appropriate mathematical equations,
the processed information allowing an observer to identify objects
that are threatening or non-threatening.
11. The weapon detection system as described in claim 10 wherein
said computer includes an embedded analog-to-digital (A/D)
converter linked directly to a preprogrammed central processing
unit (CPU) for synchronous averaging and linear filtering to yield
the decay curve in a range of 1 to 10 microseconds following
completion of the transmission of the step pulses, the time of
transmission of the step pulses in a range of 0.1 second to 2
seconds.
12. The weapon detection system as described in claim 10 wherein
said computer is used for comparing the time constant of the decay
curve of the metal object under observation with similar values of
time constants of other metal objects contained in a data base of
said computer to predict the nature of the metal object under
observation and allow an observer to identify the metal object as
being threatening or non-threatening.
13. The weapon system as described in claim 10 wherein said
transmitter transmits step pulses causing eddy currents to flow in
more than one metal object under observation at the same time and
said computer digitally processing scattered signals for defining
time constants of decay curves of the metal objects thereby
providing an accurate determination of a relationship to the
conductive properties of the metal objects under surveillance and
the sizes and shapes of each metal object.
14. The weapon system as described in claim 10 wherein said
transmitter sends large amplitude, long period, square waves of
switched dc current, the switching of the current inducing eddy
currents to flow in a metal object carried by a person or stored in
a storage container, luggage and the like.
15. The weapon system as described in claim 10 wherein the
electromagnetic step pulses are transmitted using a large loop
antenna.
16. The weapon system as described in claim 10 wherein the receiver
is a single-axis coil magnetometer.
17. A method for observing and discriminating between a concealed
weapon and other metal objects carried by a person or storied in
various carriers such as luggage, boxes, and the like, the steps
comprising:
transmitting low intensity electromagnetic step pulses outwardly
from a magnetic field generator and causing eddy currents to flow
in the metal object creating a plurality of scattered signals;
detecting the scattered signals from the eddy currents using a
receiver, the leading edges of the transmitted step pulses taking
the form of an exponentially decay curve with time constant which
can be accurately plotted; and
digitally processing with a preprogrammed computer the information
of the time constant of the decay curve, the processed information
providing an accurate relationship of the conductive properties of
the object under surveillance and the size and shape of the object
using appropriate mathematical equations, the analyzed information
allows an observer to identify objects that are threatening or
non-threatening.
18. The method as described in claim 17 further including the step
of using the preprogrammed computer and comparing the time constant
of the decay curve with similar values of time constants contained
in a stored data base of the computer to predict the nature of the
metal object and allowing an observer to identify the metal object
as being threatening or non-threatening.
19. The method as described in claim 17 wherein the transmitter is
a magnetic field generator used for transmitting low intensity
electromagnetic step pulses in a range of 20 to 50 pulses per
second.
20. The method as described in claim 17 wherein the receiver is a
fast response magnetic field sensor.
21. The method as described in claim 17 wherein the receiver is a
plurality of fast response magnetic field sensors.
Description
BACKGROUND OF THE INVENTION
(a) Field of the Invention
This invention relates to a system for detecting different types of
metal objects and more particularly, but not by way of limitation,
to a non-obtrusive weapon detection system which allows an observer
to discriminate between a handgun and other metal objects carried
on a person.
(b) Discussion of Prior Art
Heretofore there have been a variety of patents describing the use
of electromagnetic detection systems in underground mining
applications and exploration. For example, U. S. Pat. Nos.
5,185,578, 5,066,917, 4,994,747 and 5,260,660 all to Stolarczykz,
describe methods and apparatus for detecting underground
electrically conductive objects, ore zones, etc. using transmitted
electromagnetic energy. Both downhole receivers and downhole
transmitters are used in these detection systems.
In U.S. Pat. No. 4,978,920 to Mansfield el al., a magnetic field
screen is described. The screen is developed by a coil surrounded
by a set of electrical conductors in a specific region in space.
U.S. Pat. No. 4,959,559 to Ziolkowski discloses the use of wave
propagation equations for producing localized pulses of wave
energy. U.S. Pat. Nos. 4,821,023 and 4,866,424 to Parks describe
state-of-the-art walk-through metal detectors using electromagnetic
waves for detecting weapons.
None of the above mentioned patents describe the use of a
non-obtrusive weapon detection system incorporating the recognition
of specific metal objects through the determination of a
characteristic time constant and comparison of this value with a
data base containing corresponding values for a wide variety of
metal objects. The pulses cause an eddy current to flow in a metal
object on a person with a resultant scattered signal which allows
an observer to identify objects that are threatening or
non-threatening.
SUMMARY OF THE INVENTION
In view of the foregoing, it is a primary object of the present
invention to provide an innovative and non-obtrusive weapon
detection system which can discriminate between different types of
weapons and other objects carried on a person, carried in
briefcases and luggage, in packaging, storage containers and any
other transportation means where handguns and weapons may be
smuggled or hidden.
Another object of the system is a person under surveillance need
not be aware that he or she is being monitored. The system provides
for detection without invading a person's privacy.
Still another object of the subject weapon detection system is
unlike today's obtrusive metal detectors, a person is not required
to turn over other metal objects carried on the person or to walk
through a portal in order to determine the presence of a
potentially dangerous metal object.
Yet another object of the invention is the system is designed for a
high probability of weapon detection with a low false alarm rate.
The weapon detection system is capable of discriminating a
concealed weapon from other metal objects such as key rings, coins,
jewelry, belt buckles, etc.
Another object of the invention is it's use in a variety of
applications such as in the prevention of armed robbery in
high-risk business establishments such as convenience stores,
restaurants, gas stations, taxi cabs and buses, law enforcement,
security systems for large and small gathering places as in banks,
airports, schools, public building and courthouses, prisons, crowd
control areas, military uses, particularly in peacekeeping
operations, and other requirements where a non-obtrusive weapon
detection system is required.
The subject weapon detection system includes a magnetic field
generator for transmitting low intensity electromagnetic step
pulses, similar to a series of step functions, and causing eddy
currents to flow in a metal object carried by the person. The eddy
currents create a scattered signal that is detected by one or more
fast response magnetic field sensors. The eddy currents take the
form of an exponentially decaying transient magnetic field with a
duration of a few tens of microseconds. This "decay curve" provides
a basic observable, namely, the time constant of the current decay.
A computer is used to process the information of the time constant
of the decay curve. The analyzed information allows an observer to
identify objects that are threatening or non-threatening.
These and other objects of the present invention will become
apparent to those familiar with weapon detection systems and the
use of low intensity electromagnetic pulses created as a series of
step functions and showing novel construction, combination, and
elements as described, and more particularly defined by the
appended claims, it being understood that changes in the precise
embodiments to the herein disclosed invention are meant to be
included as coming within the scope of the claims, except insofar
as they may be precluded by the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings illustrate complete preferred embodiments
of the present invention according to the best modes presently
devised for the practical application of the principles thereof,
and in which:
FIG. 1 is a schematic of the electromagnetic weapon detection
system.
FIG. 2 is a more detailed schematic of the detection system
including the transmitter and receiver electronics and transmitter
and receiver antennas.
FIG. 3 illustrates a plurality of time-decay curves of five metal
objects which have been illuminated and are under observation.
FIG. 4 illustrates an example of a signal scattered from a 22
caliber handgun. The voltage amplitude of the signal has been
transformed to a logarithm of the voltage amplitude.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Basic Theory for Electromagnetic Weapons Detection &
Identification
The detection and identification of concealed weapons as described
herein uses low frequency electromagnetic fields and is based on an
appropriate solution of Maxwell's equations. The problem is that of
computing the magnetic field scattered from a small conductive mass
when it is energized with a low frequency electromagnetic field.
The subject has been analyzed in detail by Kaufman and Keller as
published in reference Kaufman, A. A., and Keller, G. V., 1985,
Inductive Mining Prospecting, Part I: Theory: Elsevier, Amsterdam
620 pp. Rather than repeat the details of the developments
described in this publication, selected results pertinent to the
problem at hand as to the identification of concealed weapons are
described herein.
Starting with a rough approximation to the problem, it is assumed
that a weapon has the form of a sphere with radius "" and a
conductivity ".sigma.". The weapon is illuminated with an
electromagnetic wave in the form of step pulses. When the weapon is
illuminated, an instantaneous step change occurs in the amplitude
of the magnetic field vector. The electromagnetic field is
considered essentially planar in the vicinity of the weapon. The
strength of a primary field in the vicinity of the weapon (that is,
the magnetic field if no weapon were present) is defined as
"H.sub.0 ".
Using a quasi-static solution of Maxwell's equations, we find that
the primary magnetic field around the sphere is increased by an
amount due to currents induced to flow within the sphere given by
the following three equations (Kaufman and Keller, 1985, eq. 3.90
and 3.91, p. 166): ##EQU1## where "E.sub..rho..sup..alpha. ",
"B.sub.R .sup..alpha. " and "B.sub..theta..sup..alpha. " are the
only components of the anomalous fields caused by currents induced
in the sphere. These components are expressed in a spherical
coordinate system centered on the sphere, with "R" being in the
direction of the incident field. Other quantities in the equations
are defined as:
K=.mu./.mu..sub.o (the relative magnetic permeability of the
weapon).
q.sub.s =k.sub.5.sup.2 /.sigma..mu.
k.sub.s =.pi.s/.alpha. (s=1, 2, 3, . . . , .infin.)
and "t" is time (following the initiation of the current step).
The form of each electromagnetic field component is that of a sum
of exponentially decaying transients. The asymptotic behavior of
these transients is now examined. During the early part of the
transient response of the electromagnetic field (t.fwdarw.0),
Kaufman and Keller (1985, eq. 3.100, p. 169) derive expressions for
the anomalous magnetic field components. ##EQU2## where
.alpha.+1/.sigma..mu..alpha..sup.2. These expressions remain valid
only for times that meet the condition at .alpha.t<<1.
Computations show that with .alpha.t=0.03, the error in
approximation is only 10%.
During the late part of the transient decay, the field is almost
entirely determined by the first exponential terms: ##EQU3## where
.tau.o=.sigma..mu..alpha..sup.2 .pi..sup.2 a is a time
constant.
These results have a remarkable simplicity which is the basis for
the design of a highly effective weapons detection system. Each of
the three expressions for field components is of the form of a
product of two terms, the first term of which involves only the
geometry and magnitude of the primary field in the vicinity of the
weapon, and a second term involving a time constant, but
independent of the geometry and strength of the field incident on
the weapon. Measurement of the time constant provides a means for
weapon detection which is free of false alarms due to variations in
primary field strength.
The time constant can be determined from a plot of the transient
magnetic field on semi-log graph paper (the slope of the curve is
the time constant). The time constant can also be determined from
the expressions: ##EQU4##
The time constant is a function only of the
conductivity-permeability product of a sphere and its cross
sectional area. The time constant becomes larger with an increase
in either parameter. The time constant can be regarded as a direct
measure of the "electromagnetic scattering cross section" of the
sphere.
Weapons have a complex geometry. Therefore a question arises as to
whether or not this simplicity extends to more complicated shapes
of weapons. Kaufman and Keller (1985) extended the analysis to
axial symmetry and arrive at an expression for the scattering time
constant as follows (eq. 3,220, p. 223):
where "q.sub.1 " depends on the shape of the conductive body
The significance of the time constant in identifying a specific
weapon of an even more complicated shape is obvious. All weapons of
the same size, shape and metallic composition will be characterized
by a scattered electromagnetic field from an incident current step
will be the same, no matter what the strength of the incident field
or the distance of the weapon from the transmitter or the receiver
array. Thus, weapons can be differentiated at least within the
precision with which the time constant of the decaying magnetic
field can be determined.
Time constant for a typical weapon
The appropriate way to determine the time constant for a real
weapon is by illuminating that weapon with an electromagnetic field
using step pulses and measuring the time constant from the decaying
field. However, a preliminary estimate can be made by substituting
numbers in the expression for the time constant. For example,
assume that the conductivity-permeability product of the steel in a
weapon is of the order of 1 siemens.times.henries/square meter, the
radius of the sphere enclosing a weapon is 0.1 meter, the form
factor "q" is 10 (the more convoluted the shape of the metallic
body, the greater will be the form factor). Substituting these
numbers, we have an order of magnitude estimate for the time
constant of the hypothetical weapon as 1 millisecond.
The weapon will usually be carried on the body of a person. Human
flesh and bone is conductive, therefore, the time constant for the
field scattered from the person is compared with the time constant
of the weapon. The conductivity-permeability product for a human
body is probably of the order of magnitude of 10
siemens.times.henries/square meter. The corresponding order of
magnitude time constant for a human body is 0.1 microsecond.
These are rough estimates, and can be refined by direct
measurement. But, the two time constants are so different that
there is no likelihood that one will obscure the other.
Referring to both FIGS. 1 and 2, schematics of the electromagnetic
weapon detection system are shown having a general reference
numeral 10. The system 10 includes a transmitter 12 for
illuminating an electromagnetic signal in the form of a pulse step.
The transmitter 12 includes solid state electronics 13 with a
switch with a 10 usec risetime, a pulse former with 20 to 50 pulses
per second and rated for 20 to 60 ampere. ##STR1##
The 20 to 50 pulses per second of electromagnetic signals are shown
as dashed rings 14 emanating from the transmitter 12 toward a large
loop transmitter antenna 16 shown in FIG. 2. A solid ring 18
indicates an interrogated volume. For example, the interrogated
volume 18 might be an entrance area into a store or bank, a walkway
between buildings, a departure area at an airport, the boarding
area of a bus, etc.
A temporal change in the electromagnetic wave field causes current
to flow in a metal object 20. The metal object 20 is also refered
to as a target. Scattered electromagnetic signals are shown as
dashed rings 22 emanating from the metal object 20. The scattered
electromagnetic signals 22 have the form of branches of an
exponential curve, with a time constant that is a function of the
size, shape and material composition of the metal object 20 under
observation. The scattered electromagnetic signals 22 are detected
by a loop receiver antenna 24 or other fast response magnetic field
sensors acting as a magnetometer or by an array of magnetometers.
From the antenna 24, a signal processing unit 26 with receiver
electronics 28 conditions the scattered signals 22 in preparation
for digital processing of the information. This information is
processed using a computer 30 with A/D converter or equivalent
dedicated digital circuitry and wherein the time constant of the
metal object 20's decay curve is analyzed using a software code
capable of determining time constants and the various scattering
cross-sections of the objects under observation.
The signal processing system using the computer 30 must accomplish
three thing: 1) accurate determination of the scattered signal 22;
2) computation of the time constant embedded in the signal 22; and
3) determination of the nature of metal object 20 scattering the
signal 22 with that particular time constant.
1) Accurate determination of the signal 22 can best be accomplished
through synchronous detection (averaging) of several signals caused
by repeated application of the excitation field, along with
application of appropriate linear and non-linear digital filtering
schemes for which the design may be specific to the site where the
system is installed. Rapid determination of the meaning of a signal
is of the essence in an effective security system, with a need to
acquire and process data in a very short time being an essential
part of the design of the system. Ideally, data acquisition and
processing of the signals should be accomplished in a time not
exceeding one second, though in some applications, times as long as
several seconds may be appropriate. For a system operating at a
basic frequency of 50 Hertz, 20 step excitations can be achieved in
one-tenth of a second. It is expected that synchronous addition and
linear filtering will be accomplished with an embedded
analog-to-digital (A/D) converter linked directly to a
preprogrammed central processing unit (CPU) to yield a signal curve
like those shown in FIG. 3 within at most a few microseconds
following completion of the excitation cycle of the system, a time
which will range from as little as 0.1 second to as great as
several seconds.
2) Accurate determination of a time constant will be accomplished
by post-processing after a cycle of excitation has been completed
and an averaged signal resides within digital memory of the digital
system. One approach to determining the time constant is shown by
data in FIG. 4. Here, synchronously stacked signals from a metal
target have been replotted (algebraically transformed) by using the
logarithm of the signal strength, rather than the signal strength
itself. With this transformation, an exponential decay curve now
appears as a straight line, with the slope of that line being the
time,constant. One easy approach to determining time constant is by
fitting the transformed data, that is, a signal in which the signal
amplitude is converted to its logarithm, with a best-fit
one-parameter linear correlation function. The one parameter in the
fit is the slope, which is the parameter to be used in identifying
the nature of the scatterer. It is anticipated that this analysis
will require less than 0.1 second.
There are many standard programs for determining such one-parameter
correlation functions; the approach just described is illustrative
of any of these, and should be considered only as an example of how
this step in signal analysis can be accomplished.
3) With the numerical value for the time constant associated with a
detected target, it is next necessary to determine the likely
nature of that target, again in a timely manner. One approach is to
compile a catalog of time constants for various weapons and of
other metal objects which can be carried on the person, in a
storage unit, etc. This catalog being referred to herein as a "data
base". In this data base, messages would be stored describing the
potential threat corresponding to a given time constant. The
"address" of each cell in the data base would be a number which is
the integer equivalent of the time constant which has been
determined in step 2 above. In this approach, searching even as
extremely large data base will require only tens of
milliseconds.
There are many standard programs for searching a one-parameter data
base. The approach just described is illustrative of any of the
these and should be considered only as an example of how the nature
of the threat represented by a given time constant can be
determined in a timely manner.
In FIG. 3, an example of the scattering properties observed for a
variety of five different metal objects using step pulses, such as
a Heaviside pulse, are illustrated. Note the peak amplitude of the
curves of the five different metal objects are similar and in this
example slightly less than 1.9 volts. As the decay of each curve
takes place, each object under observation takes on it's own
recognizable curve characteristics. Curve 32 for an 18 inch pipe
for example has a flatter curve than curve 34 for a Browning 9 mm
pistol and curve 36 for a 3.9 inch cannonball. Curve 38 for a 4
inch can and curve 40 for a 6.6 inch copper sphere illustrate the
shapes of decay curves under observation. This figure shows how the
different metal objects have distinct decay transients that are
measurable and wherein the time constant of each object is a direct
measure of the electromagnetic scattering cross section of object's
profile.
In FIG. 4, an example of a signal 42 scattered from a 22 caliber
handgun is illustrated with the voltage amplitude of the signal 42
having been tranformed to a logarithm of the voltage amplitude.
This transformation has changed the form of the transient decay
curve as illustrated in FIG. 3 to that of an approximate straight
line with a slope equal to the time constant of the decay of the
signal 42. Note that this behavior is distorted at times before a
few tens of microseconds by the limited rate of response of the
magnetic field sensor used in this particular experiment. Also, the
decay curve, when at times longer 3.5 milliseconds (3500
micro-seconds), becomes distorted as the signal strength drops to a
point where extraneous noise is significant. The essence of
practical design of equipment is to obtain accurate information
about the strength of the scattered signal over the time range from
a few tens of microseconds to a few milliseconds.
While the invention has been particularly shown, described and
illustrated in detail with reference to the preferred embodiments
and modifications thereof, it should be understood by those skilled
in the art that the foregoing and other modifications are exemplary
only, and that equivalent changes in form and detail may be made
therein without departing from the true spirit and scope of the
invention as claimed, except as precluded by the prior art.
* * * * *